In internal combustion engines, it is desirable to vary compression ratio during engine operation Compression ratio strongly affects in-cylinder processes and provides an exceptional degree of control over engine performance.
Conventional engines, however, have fixed compression ratios. Their performance is a compromise between conflicting requirements.
The provision of variable compression ratio in diesel engines improves exhaust emission characteristics, overall fuel efficiency, cold startability and multi-fuel capability. It provides control over peak cylinder pressures and therefore permits considerable increase of specific power output through supercharging without sacrificing engine durability.
In spark ignited engines the variability of compression ratio enables operation at the knock limit throughout the whole speed and load range. This results in part-load thermal efficiency improvement relative to conventional engines whose fixed compression ratios are restricted to relatively low values required to ensure knock free wide open throttle operation.
Numerous mechanisms for varying compression ratio in internal combustion engines have been proposed in the prior art. The majority of those mechanisms vary compression ratio by varying clearance volume. This is primarily achieved by (1) altering the volume of an auxiliary combustion chamber; (2) altering the distance between the cylinder head and the axis of rotation of the crankshaft; (3) altering the effective length of the connecting rod or; (4) altering the distance between the wrist pin axis and the top face of the piston.
So far, only two VCR mechanisms have attained series production. Both have, for decades, been used in standardized, single cylinder ASTM-CFR cetane and octane rating engines. The cetane rating engine utilizes a handwheel operated screw adjustment mechanism to vary the position of a movable endplate in a cylindrical prechamber. The octane rating engine employs a motor driven screw adjustment mechanism to vary the position of the cylinder head cylinder liner assembly relative to the axis of rotation of the crankshaft.
Prior art also includes U.S. Pat. No. 2,742,027 which discloses a two part VCR piston assembly. The inner member of this assembly is linked to the crankshaft in the conventional manner using a wrist pin and a connecting rod. The outer member which serves as the piston crown and carries the piston rings is axially movable relative to the inner member within predetermined limits. Any change in the relative position of both members alters the distance between the top face of the piston and the wrist pin axis and, therefore, affects compression ratio in the cylinder in which the whole VCR piston assembly operates. A hydraulic system utilizing engine lubricating oil supplied through the connecting rod is incorporated in the VCR piston assembly and automatically controls compression ratio as a function of engine speed and load.
Despite extensive development, the VCR piston has not been commercially successful due to engine reliability and performance problems associated with this design. Reliability problems arise because of coking of the lubricating oil throughout the piston assembly. Such coking causes malfunctioning of the compression ratio control system and leads to engine failures. The performance problem stems from the adverse effect that combustion chamber geometry changes associated with compression ratio variation have on the combustion process, especially on exhaust emissions and fuel efficiency.
However, the principal reason why the VCR piston in particular and VCR mechanisms in general have not been commercially successful is that multi-cylinder engines incorporating VCR technology are inherently mechanically complex.
The present invention belongs to a category of VCR mechanisms which vary compression ratio by altering the phase relation between two pistons operating in interconnected cylinders.
In engines utilizing VCR mechanisms of this category, compression ratio is defined as maximum combined cylinder volume divided by minimum combined cylinder volume. If, for example, the two pistons operate in the cylinder of an opposed piston engine, combined cylinder volume is defined as the total volume between those two pistons at any instant. If the two pistons operate in separate cylinders interconnected through a transfer port, combined cylinder volume is assumed to be composed of individual cylinder volumes at any instant and the transfer port volume.
Compression ratio is maximum when both pistons move in phase, reaching their respective top dead center positions simultaneously. Any phase shift from that maximum compression ratio phase relation alters compression ratio by changing both the maximum and the minimum combined cylinder volume. Said phase shift is measured in terms of crank angle relative to a phase relation which corresponds to the two pistons reaching their respective top dead center positions simultaneously. The value of the phase shift angle can be arbitrarily assigned a positive or a negative sign to indicate whether the movement of one or the other piston is advanced or delayed relative to the combined cylinder volume changes. Within the phase shift angle range of practical interest in VCR engines, the greater the absolute value of the phase shift angle, the lower the compression ratio.
The following prior art discloses mechanisms which vary compression ratio by altering the phase relation between two pistons operating in interconnected cylinders. These two pistons are generally linked to separate crankshafts and their phase relation is altered by varying the phase relation between those crankshafts. A multitude of such piston pairs may, of course, be incorporated in an engine.
U.S. Pat. No. 1,457,322 discloses a two crankshaft engine which employs a VCR mechanism comprised of helical gears, some of which are axially movable. Specifically, this mechanism includes two pairs of helical gears which couple the two crankshafts to an axially movable phase shaft. Each pair of those gears consists of a helical gear mounted on a crankshaft and, engaged therewith, a helical gear mounted on the axially movable phase shaft. The crankshafts are situated side by side and the phase shaft transversely thereto. Helix angles and directions of helixes of those gears are arranged to alter the phase relation between the two crankshafts in response to axial displacement of the phase shaft. The principal advantage of this VCR mechanism is mechanical simplicity. However, the location of the phase shaft in an extension of the crankcase results in a significant increase of engine length. In addition, the operation of helical gears on nonparallel shafts considerably reduces their load carrying capacity and/or useful life.
U.S.S.R. Pat. No. 300643 discloses a VCR mechanism which employs helical splines to vary the phase relation between two crankshafts of an opposed piston engine. The mechanism is incorporated in a transverse shaft geared to both crankshafts and consists of two separate helical spline couplings which couple two segments of the said shaft to an axially movable member located coaxially between those segments. The helix angles and directions of helices of the splines are arranged to vary the phase relation between the two segments of the transverse shaft and, consequently, between the two crankshafts, in response to axial displacement of the movable member. The principal disadvantage of this VCR mechanism is the mechanical complexity of the whole crankshaft phasing system. Due to the sliding fit requirement and the resultant presence of backlash between mating surfaces, the durability of spline couplings which are subject to heavy alternating loads is also compromised.
U.S. Pat. No. 3,961,607 discloses a two crankshaft VCR engine incorporating a planetary gear set in the crankshaft phasing system. The phase relation between crankshafts is varied by rotating the planetary gear carrier around its axis. This VCR mechanism is mechanically complex.
In 1984, an article authored by C. M. Bartolini, V. Naso and this inventor was published in a Polish journal, "Archiwum Termodynamiki", Vol. 5, No. 2. It disclosed a VCR mechanism employing two pairs of helical gears which couple the two crankshafts of the engine to an axially movable phase shaft. Each pair consists of a helical gear mounted on a crankshaft and, engaged therewith, a helical gear mounted on the movable phase shaft. The crankshafts are situated side by side and the phase shaft parallel thereto. The two crankshaft mounted gears are located at the opposite ends of the engine. Helix angles and directions of helices of the VCR mechanism gears are arranged to alter the phase relation between the two crankshafts in response to axial displacement of the phase shaft. In order to accommodate changes in the relative axial position of those gears associated with compression ratio variation, the face width of the crankshaft mounted gears is greater than the face width of the phase shaft mounted gears. This particular parallel configuration of the crankshafts and the phase shaft is favorable from the standpoint of gear durability and load carrying capacity but results in a considerable increase of engine length.